Supplying catalyst to a polymerization reactor and achieving high catalyst efficiency while minimizing undesired consequences has been a challenge for many commercial processes. The problems encountered depend upon the form of the catalyst (i.e., solid, size of particles, liquid, type of solvent, etc.) and the polymerization process being used. The problems encountered can arise from catalyst degradation, poor control of catalyst feed rate, plugging of feed lines, poor mixing of the catalyst with monomers and other polymerization media, introduction of undesired quantities of carrying medium to the process, poor solubility to the polymerization medium or carrying solvent, and concerns of residual solvent in products. In particular, some catalysts suffer from prolonged contact with the hydrocarbons used as carriers in the process.
Most polymerization catalysts are either supported, heterogeneous catalysts, or unsupported homogeneous catalysts (described more below). Supported, heterogeneous catalysts are used in slurry and gas phase polymerization reactors. The active ingredients of such catalysts are supported on solid, insoluble supports and thus insoluble in any solvent. Typically, such supported catalysts are delivered or “passed” into a process line and/or reactor in a dry powder form or as a slurry in a solvent. Gas-phase fluidized bed polymerization reactors often employ dry catalyst delivery systems, sometimes utilizing a gas carrier. Slurry reactors often employ some type of solvent to deliver the heterogeneous catalyst.
But polymerization processes that employ solvents such as in slurry and solution polymerization present particular problems. Unlike the case for gas-phase polymerization reactors, it is undesirable to inject a gas when delivering catalyst into slurry or solution polymerization reactors as the excess gas could cause operability problem. Further, some supported heterogeneous catalysts suffer from prolonged contact with the solvent used to deliver them and result in low catalyst efficiency. Passing a purely solid, powder catalyst to a slurry or solution reactor can also present problems as the productivity of the system may be low.
Homogeneous catalysts are used in solution polymerization processes. Many olefin polymerization processes are carried out in the presence of an inert liquid organic solvent, and the polymer produced is dissolved in that inert organic solvent. In an olefin solution polymerization, catalysts are typically dissolved in a carrying medium and delivered into the polymerization reactor in a solution form. The catalyst solution is then mixed with monomers and other polymerization medium and the polymerization takes place in the liquid state. The catalyst carrying medium can be the same as the solvent used for polymerization, or different types of solvents with better solvency may be used. In some cases there is a problem of the catalyst, though homogeneous, being poorly soluble.
Aliphatic hydrocarbon solvents are typically used for solution polymerization of olefins. In contrast, an aromatic solvent is typically used as catalyst carrying medium due to poor solvency of catalysts in aliphatic hydrocarbon solvents. It is recognized that the use of aromatic solvent is advantageous since good solubility improves catalyst utilization efficiency. However, there are other concerns regarding the use of aromatic solvents such as toluene and xylene. Use of aromatic solvent can add additional requirements/cost in solvent separation from the high molecular weight polymer product and the solvent recovery and recycle back to the polymerization reactor. Prolonged exposure of catalyst to a carrying medium such as a hydrocarbon solvent might result in catalyst deactivation or cause process deficiencies.
It would be desirable to use non-aromatic solvents to deliver catalyst to a solution polymerization reaction. US 2015/0094434 disclosed the method of using aliphatic hydrocarbon solvent to dissolve metallocene catalysts. However, the catalyst concentration in the aliphatic hydrocarbon solvent is limited to a range from 0.02 to 0.6 mmol per liter of solvent due to poor solvency of the aliphatic hydrocarbon solvent of the catalysts in that disclosure. In that disclosure, it was found that a suspension of the catalyst resulted in poor polymer productivity and polymer yield.
The polymer production rate in a polymerization reactor is dependent on the catalyst feed rate. To increase production, a higher feed rate is needed. But for catalyst that is only soluble in aromatic solvents, this means adding more aromatic solvent in the reactor which is undesirable; alternatively, for catalyst only slightly soluble in aliphatic hydrocarbon solvents, larger feed pumps, higher levels of solvent, and larger feed lines are needed, adding cost to the production. If a means of delivering a high concentration of catalyst is found that would reduce the amount of solvent needed, aromatic or otherwise, smaller pumps and less solvent could be used, thus providing an economic benefit. Thus, an improved method of delivering powder catalyst to a solution polymerization process/reactor is needed.
Relevant patents and publications also include U.S. Pat. Nos. 3,012,024; 3,790,036; 3,876,602; 4,610,574; 4,774,299; 5,955,554; 5,714,424; US 2002/0034464; US 2015/0094434; EP 878 483 A1; WO 92/16747; and WO 96/021684.